The invention relates to a supplementary rear-wheel steering for motor vehicles having a control or regulating device which actuates the steering of the rear wheels as a function of the steering angle of the manually-controlled front steering wheels (main steering) by means of a servomotor, servo-assisted by a double-acting piston-cylinder unit and as a function of further parameters (e.g. transverse acceleration, and/or travelling speed of the vehicle, and/or by comparison of set and actual values of the rear wheel steering angle).
Supplementary rear-wheel steering systems of this general kind are in principle known from Japanese Published Specification 62-199 568.
To facilitate maneuvering operations, the rear wheels can be deflected in the opposite direction to the front wheels during very low travelling speeds in order to reduce the turning circle of a vehicle.
At higher travelling speeds, provision is made to deflect the rear wheels in the same direction as the front wheels but with a clearly smaller steering angle, in order to increase the slip angle of the rear wheels at the very beginning of cornering and thereby improve directional stability.
It is in addition fundamentally known to control the supplementary rear-wheel steering as a function of additional parameters in order, for example, to automatically mitigate effects of side winds or also swerving movements of the vehicle.
In the case of such highly developed supplementary rear-wheel steering systems, a computer control is very often necessary. However, in the case of a malfunction of the computer, it must be possible to make a switch to a computer-independent emergency control function for the rear-wheel steering. In this connection, the automatic return of the rear wheels to their neutral central position as soon as a malfunction of the computer is detected has been envisaged.
However, this concept has the considerable disadvantage that an extremely critical driving situation may occur in certain circumstances, if the returning of the rear wheels is effected in driving situations within the threshold region of the controllability of the vehicle. Furthermore, the driver may be dangerously surprised, by a sudden returning of the previously deflected rear wheels to the central position even in driving situations which are not dangerous per se.
It is therefore the object of the invention to create a supplementary rear-wheel steering which, in the event of a failure or a malfunction of the computer-controlled control (regulating device) guarantees particularly high driving safety in a manner which is simple in terms of construction.
This object is achieved by having a steering linkage element of the rear wheel steering coupled to a further double-acting hydraulic piston-cylinder unit and by having the piston-cylinder units of the front wheel steering and of the rear-wheel steering identical with respect to their swept volumes. Two separate lines are arranged between the two piston-cylinder units to connect the chambers of the two piston-cylinder units directly to one another. A further connection is established between the two lines by a valve which is open only while the control or regulating device is operating correctly and in the case of control malfunction, the valve closes and couples the piston-cylinder unit of the rear-wheel steering to the piston-cylinder unit of the front-wheel steering, while the servomotor of the rear wheel steering simultaneously switches over to a self-locking-free state.
The invention is based on the general idea of effecting a hydraulic positive coupling between front wheel and rear wheel steering in the event of an error detection in the system of the regulating control. At the same time, the fact that the swept volumes of the two piston-cylinder units are the same, ensures that the rear wheel steering can be actuated over the maximum possible steering range of the rear wheels when the piston of the further piston-cylinder unit, associated with the main steering, is moved from one extreme position to the other.
A particular advantage of the invention consists in the fact that when the positive coupling between front-wheel and rear-wheel steering comes into effect, an independent steering movement of the rear wheels, which would under certain circumstances be extremely unexpected, does not take place. Rather, the rear wheels are from then on only steered together with the front wheels.
If the positive coupling between front-wheel steering and rear-wheel steering takes place at a time at which exact synchronism between front wheels and rear wheels does not exist and, accordingly, the vehicle could only follow a straight road by running as it were askew (where the longitudinal axis of the vehicle is slightly oblique to the driving direction), then synchronism is obtained by having two bypasses of the front-wheel steering being actuated in such a way that the piston of the further piston-cylinder unit reaches an extreme position.
Here, it is necessary to steer to the left if, in straight-ahead travel while askew, the longitudinal axis of the vehicle points to the right in the driving direction. In the opposite case, it is necessary to steer to the right. If the driver does not take account of this, complete synchronism can be enforced by steering strongly in one direction and then strongly in the other direction in succession. The front and rear wheels thereupon each simultaneously assume their neutral central position, i.e. a slip-free straight-ahead travel. Then the longitudinal axis of the vehicle will lie in the driving direction.
The servo-motor can have its output and input be set to the unpressurized condition by means of valves which connect the inputs and outputs to a reservoir and which switch over automatically upon error detection in the regulating control so as to be freely moveable, i.e., in a self-locking free state.
It is also advantageous to keep the two piston-cylinder units and the line system between the latter permanently under pressure in order to be able to detect immediately any leaks by means of continuously operating pressure transducers located in these lines.
Additional auxiliary control of the servo-motor controlling rear-wheel steering is obtained by applying pressure or venting of the servo-motor when malfunction conditions do not exist.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.